1. Field of the Invention
The present invention relates to an apparatus for controlling a pulley side-pressure to be applied to a drive pulley and a driven pulley of a belt type continuously variable transmission mechanism which is provided, in series with a starting clutch, in a transmission of a vehicle having a function of stopping engine idling so that an engine is automatically stopped under given conditions when the vehicle is at a standstill.
2. Description of Related Art
Conventionally, the pulley side-pressures to be applied to a drive pulley and a driven pulley of a belt type continuously variable transmission are controlled based on a belt transmission torque which corresponds to an output torque of an engine, the control being made such that slip does not occur to a belt. The output torque of the engine is calculated from the rotational speed and a negative suction pressure of the engine.
In a vehicle having a function of stopping engine idling, at the time of vehicle start-up from the state of engine stopping, the engine is started from a state in which the negative suction pressure has not been generated. Therefore, until the rotational speed of the engine increases to a certain degree and the negative suction pressure consequently becomes large enough, the engine output torque which is calculated from the rotational speed of the engine and the negative suction pressure becomes larger than an actual value. As a result, the pulley side-pressure becomes excessive, and the durability of the pulleys and the belt is badly affected. Further, a friction loss becomes large at a point of the belt""s coming into contact with the pulley and a point of the belt""s going out of contact with the pulley, resulting in a bad effect on the specific fuel consumption.
In view of the above points, the present invention has an object of providing an apparatus for controlling a pulley side-pressure in which the pulley side-pressure can be prevented from becoming excessive at the time of vehicle start-up from the state of engine stopping.
In order to attain the above and other objects, the present invention is an apparatus for controlling a pulley side-pressure to be applied to a drive pulley and a driven pulley of a belt type continuously variable transmission mechanism which is provided, in series with a starting clutch, in a transmission of a vehicle having a function of stopping engine idling so that an engine is automatically stopped under given conditions when the vehicle is at a standstill. The apparatus comprises: means for calculating a first belt transmission torque which corresponds to that output torque of the engine which is obtained from a rotational speed and a negative suction pressure of the engine; means for calculating a second belt transmission torque which corresponds to that transmission torque of the starting clutch which is obtained from an engaging force of the starting clutch; and means for controlling a pulley side-pressure based on whichever is smaller between the first and second belt transmission torques at a time of vehicle start-up from a state of engine stopping.
Even if the output torque of the engine is large, the belt transmission torque, i.e., the transmission torque of the continuously variable transmission mechanism will not become larger than the transmitting toque of the starting clutch. Therefore, even if the pulley side-pressure is controlled based on whichever is smaller between the first belt transmission torque corresponding to the engine output torque and the second belt transmission torque corresponding to the transmission torque of the starting clutch, belt slipping does not occur.
According to the present invention, even if that output torque of the engine which is calculated by the rotating speed and the negative suction pressure of the engine becomes larger than the actual value, the pulley side-pressure is controlled to a value corresponding to the transmission torque of the starting clutch. The pulley side-pressure can thus be prevented from becoming excessive.
The friction coefficient of a clutch disc of the starting clutch varies from clutch to clutch and further varies or deteriorates with the lapse of time. Therefore, the actual transmission torque of the starting clutch sometimes exceeds an ordinary transmission torque which is obtained by the engaging force. In such a case, if the pulley side-pressure is controlled to a value corresponding to the ordinary transmission torque, the belt gives rise to slipping. Therefore, the transmission torque of the starting clutch to be obtained by the engaging force of the starting clutch shall be set to a value obtained by multiplying an ordinary transmission torque of the starting clutch (7) by a predetermined safety factor. As a result, if the pulley side-pressure is controlled based only on the transmission torque of the starting clutch, the pulley side-pressure sometimes becomes excessive as compared with the actual belt transmission torque. As a solution, in the present invention, when the belt transmission torque corresponding to the output torque of the engine is smaller than the belt transmission torque corresponding to the transmission torque of the starting clutch, the pulley side-pressure is controlled by the belt transmission torque corresponding to the output torque of the engine.
In case the starting clutch is disposed on the output side of the belt type continuously variable transmission mechanism, there is transmitted a torque to the starting clutch, the torque being of a value obtained by multiplying the input torque (belt transmission torque) of the continuously variable transmission mechanism by the reduction ratio of the continuously variable transmission mechanism. Therefore, the belt transmission torque becomes a value which is obtained by dividing the transmission torque of the starting clutch by the reduction ratio of the continuously variable transmission. However, at the time of vehicle start-up from the state of the engine stopping, the belt transmission torque is partly used or consumed for rotating the stationary driven pulley. The torque to be transmitted to the starting clutch is thus reduced by the above-described amount of consumption. Therefore, preferably, the second belt transmission torque is set to a value which is obtained by adding a first value and a second value together wherein the first value is that transmission torque of the starting clutch which is obtained from the engaging force of the starting clutch as divided by a reduction ratio of the belt type continuously variable transmission and the second value is that torque corresponding to an inertia of the driven pulley which is required to rotate the stationary driven pulley.